This invention relates to a push-button switch, and more particularly to a push-button switch of a water- and dust proof structure having an enclosing insulating case.
In recent years, electronic appliances have been reduced in size and weight, and simultaneously the density of parts mounted on a printed circuit board has been increased progressively. Accordingly, a push-button switch of the face mounted type is frequently used which is first placed at a predetermined location of a printed circuit board to which cream solder has been applied in advance and is then heated in a reflow furnace or the like in order to solder connecting terminals thereof to a wiring pattern of the printed circuit board. As one of such push-button switches of the face mounted type, a push-button switch of a water- and dust-proof structure wherein contact portions are enclosed in order to improve the reliability has been already provided and is disclosed, for example, in Japanese Patent Laid-Open No. 61-142617.
FIGS. 10 and 11 show an exemplary one of conventional push-button switches of such a water- and dust-proof structure. The push-button switch shown includes two metal pieces embedded in a spaced relationship in a heat resisting insulating case 1 having an opening formed at the top thereof. One of the metal pieces is partially exposed from an inner face of a bottom wall of the insulating case 1 to form a fixed contact 2 and the other metal piece is also partially exposed from the bottom wall inner face of the insulating case 1 to form another fixed contact 3 while end portions of both the metal pieces are exposed outside and extend outwardly from a side wall of the insulating case 1 to form a pair of connecting terminals 4. A flattened bowl-shaped spring disk 5 serving as a movable contact is placed on the fixed contact 2 such that, when the push-button switch is not operated, it is spaced by a little distance from the fixed contact 3 at the center of the insulating case 1. A stem 6 is received for up and down movement in the top opening of the insulating case 1. The stem 6 is made of a like material to the insulating case 1 and has a semispherical projection 6a formed on a lower face thereof and normally held in contact with the spring plate 5. The stem 6 is covered at an upper face thereof by a heat resisting film 7 for water- and dust-proof which closes the top opening of the insulating case 1. A holding plate 8 is located on an upper face of the heat resisting film 7 and secured to the insulating case 1 for limiting upward movement of the stem 6. The holding plate 8 is made of, for example, a solder-plated stainless steel plate and has a perforation 8a formed at a central portion thereof. Thus, an operating area 6b of the stem 6 covered by the heat resisting film 7 extends upwardly through the perforation 8a of the holding plate 8. The holding plate 8 partially extends laterally and then downwardly to form a grounding terminal 9.
In mounting the push-button switch having such a construction as described above, it is first placed at a predetermined location of a wiring pattern 11 on a printed circuit board 10 to which cream solder has been applied in advance and is then heated in a reflow furnace to a temperature higher than 200.degree. C. so as to melt the cream solder. Consequently, after cooling of the push-button switch, the connecting terminals 4 and the grounding terminal 9 are connected to the wiring pattern 11 by the solder 20 as seen in FIG. 11. If the operating portion 6b of the stem 6 of the push-button switch which has been mounted on the printed circuit board 10 in this manner is manually operated to depress the stem 6, the spring plate 5 is resiliently yielded downwardly until it is contacted with the fixed contact 3, thereby electrically interconnecting both the fixed contacts 2, 3 to establish a closed state of the push-button switch. Then, if the pressing force upon the operating portion 6b of the stem 6 is removed, then the spring plate 5 will be spaced away from the fixed contact 3 by its own resiliency, thereby restoring an open state of the push-button switch.
By the way, in the push-button switch of a water- and dust-proof structure described above, fine gaps 4a are defined between the insulating case 1 and an embedded portion of each of the connecting terminals 4 which are insert-molded in the insulating case 1. Accordingly, when the push-button switch to be mounted is heated in a reflow furnace or the like and consequently air within the insulating case 1 expands, the air will flow out through the gaps 4a. Then, as the temperature of the push-button switch drops after soldering, the air within the insulating case 1 contracts, but thereupon flux contained in the cream solder may enter and close the gaps 4a thereby to obstruct external air from flowing into an internal spacing of the push-button switch through the gaps 4a. If this actually occurs, the pressure of the air within the push-button switch will be lower than the pressure of air outside the push-button switch, and consequently the stem 6 will have an initial position lower than an aimed initial position therefor and hence have an undesirably small stroke of operation, resulting in deterioration of the operating force characteristic of the push-button switch.